with +1.2V Buffered Reference

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11

Applications Information

Power-Supply Considerations

The MAX4036–MAX4039 operate from a single 1.4V

(MAX4036/MAX4038) or 1.8V (MAX4037/MAX4039) to

3.6V supply. A high amplifier power-supply rejection

ratio of 82dB and the excellent reference line regulation

allow the devices to be powered directly from a decay-

ing battery voltage, simplifying design and extending

battery life. The MAX4036–MAX4039 are ideally suited

for low-voltage battery-powered systems. The Typical

Operating Characteristics show the changes in supply

current and reference output as a function of supply

voltage.

Power-Up Settling Time

The MAX4036–MAX4039 typically require 0.25ms to

power-up. During this startup time, the output is inde-

terminate. The application circuit should allow for this

initial delay. See the Typical Operating Characteristics

for amplifier and reference settling time curves.

Driving Capacitive Loads: Op Amps

The MAX4036–MAX4039 amplifier(s) require no output

capacitor for stability, and are unity-gain stable for

loads up to 5000pF. Applications that require greater

capacitive-drive capability should use an isolation

resistor between the output and the capacitive load

(Figure 2). Note that this solution reduces the gain and

divider with the load resistor.

Crossover Distortion

The MAX4036–MAX4039 output stages are capable of

sourcing and sinking currents with orders of magnitude

greater than the stages’ quiescent current, which is

less than 1µA. This ability to drive heavy loads with

such a small quiescent current introduces crossover

distortion as the output stage passes between sinking

and sourcing. In the crossover regions, the output

impedance of the MAX4036–MAX4039 increases sub-

stantially, thereby changing the load-driving character-

istics. The distortion can be greatly reduced by

increasing the load resistance. For applications where

low load resistance is required, bias the load such that

the output current is always in one direction, to avoid

crossover distortion.

Reference Bypassing

The MAX4037/MAX4039 reference requires no external

capacitors.

Using the MAX4036–MAX4039 as a

Comparator

Although optimized for use as an operational amplifier,

the MAX4036–MAX4039 can be used as a rail-to-rail

I/O comparator (Figures 3, 4). External hysteresis can

be used to minimize the risk of output oscillation. The

positive feedback circuit, shown in Figure 4, causes the

input threshold to change when the output voltage

changes state.

Battery Monitoring Using the

MAX4037/MAX4039 and Hysteresis

The internal reference and low operating voltage of the

MAX4037/MAX4039 make the devices ideal for battery-

monitoring applications. Hysteresis can be set using

resistors as shown in Figure 4, and the following design

procedure:

1)

Choose R3. The input bias current of IN_+ is under

100pA over temperature, so a current through R3

around 100nA maintains accuracy. The current

for R3 = 12M

Ω. 10MΩ is a good practical value.

2)

between the upper and lower thresholds. In this

MAX4038

= 1

Figure 2. Using a Resistor to Isolate a Capacitive Load from

the Op Amp

INPUT

OUTPUT

Figure 3. Hysteresis